1. Field of the Invention
The present invention relates to a transmission amplifier and in particular to an apparatus controlling a degree of peak suppression at a peak suppression unit for suppressing a peak power of an input signal of the transmission amplifier.
2. Description of the Related Art
The next generation mobile communication is premised on an adoption of the Orthogonal Frequency Division Multiplexing (OFDM) and Multiple Input Multiple Output (MIMO). When adopting these, a power consumption of a radio frequency (RF) circuit must be reduced. The RF circuit is constituted by electronic components such as an antenna, power amplifier, RF filter, and AD converter. Among these electronic components, a reduction of power consumption of a power amplifier (named as “transmission amplifier” hereinafter) is especially important.
A linear amplification is required of a transmission amplifier for use in a digital radio communication system. A signal with a large peak-to-average power ratio (PAPR) usually requires a high linearity.
FIG. 1 is a diagram showing a relationship of an input/output characteristic of a transmission amplifier with a peak of a signal. In FIG. 1, the horizontal axis is an input power of a transmission amplifier, and the vertical axis is an output power thereof.
As shown by a characteristic curve 1001 in FIG. 1, the relationship between the input power and output power changes from linear to nonlinear with an increase of the input power, with the amplifier gain gradually becoming saturated.
As shown on a lower side of FIG. 1, for a signal with a large PAPR (i.e., a signal with an average power at an operation point 1 indicated in the diagram), the transmission amplifier needs to be operated in a linear zone by allowing a large back-off power of the transmission amplifier, resulting in a large reduction in a power added efficiency (PAE). Contrarily, for a signal with a small PAPR (i.e., a signal with an average power at an operation point 2 indicated in the diagram), the transmission amplifier can be operated in high efficiency because a back-off power can be small. Note that the operation points 1 and 2 are average powers of a signal with a large PAPR and that with a small PAPR, respectively. The peak power is the maximum amplitude (i.e., the maximum input power) of each waveform. Here, the back-off power is defined as the difference between the peak power and average input power.
As a method for preventing a reduction of efficiency of a transmission amplifier for inputting and amplifying a signal with a large PAPR as described above, among proposed for example is a peak suppression method of a circuit configuration as shown in FIG. 2. The peak suppression method is configured to equip a peak suppression unit 1120 in front of a digital-to-analog (D/A) conversion unit 1130 so that the peak suppression unit 1120 suppresses a peak component of a signal output from a digital signal generation unit 1110. A peak suppression signal as a result of the peak suppression unit 1120 suppressing the peak component is converted into an analog signal by the D/A conversion unit 1130, followed by being multiplied, at a mixer 1150, by a carrier wave output from a local oscillator 1140. The modulation signal generated by the mixer 1150 is amplified by a transmission amplifier 1160, and then emitted as a radio wave from an antenna 1170.
FIGS. 3 and 4 respectively exemplify conventional circuit configuration of the peak suppression unit 1120, with FIG. 3 showing a method employing a clip, FIG. 4 showing a method employing a window function.
The peak suppression circuit of the clip method shown by FIG. 3 comprises a delay (i.e., Delay) unit 1121, an amplitude arithmetic operation unit 1122, a peak detection unit 1123, a threshold/amplitude unit 1124 and a multiplier 1125. Briefly describing the operation of the circuit, a transmission signal S(t) is input to the delay unit 1121 and amplitude arithmetic operation unit 1122. The amplitude arithmetic operation unit 1122 calculates an amplitude |S(t)| of the transmission signal S(t) and outputs it to the peak detection unit 1123 and threshold value/amplitude unit 1124. The peak detection unit 1123, having detected a peak (peak value) of the amplitude |S(t)| of the transmission signal S(t), notifies the threshold/amplitude unit 1124 of the fact.
The threshold value/amplitude unit 1124, having been input the notification signal from the peak detection unit 1123, compares the amplitude |S(t)| input from the threshold value/amplitude unit 1124 with a threshold value Vth and performs an arithmetic operation of the following expressions (1) or (2) in accordance with the comparison result, followed by suppressing the peak (i.e., the maximum amplitude) of the transmission signal S(t) to no more than the threshold value Vth:S′(t)=S(t); if |S(t)|≦Vth  (1)S′(t)=Vth/|S(t)|*S(t); if |S(t)|>Vth  (2)
The transmission signal S(t) delayed by the delay unit 1121 and the amplitude |S(t)| output from the threshold value/amplitude unit 1124 is multiplied by the multiplier 1125 and the resultant is output as a peak suppression signal.
The peak suppression circuit of the window function method shown by FIG. 4 is configured to equip a window function generation unit 1127 in place of the threshold/amplitude unit 1124, otherwise the same configuration as the peak suppression circuit of the clip method shown by FIG. 3. The same component sign is assigned to the same constituent component as one shown in FIG. 3.
The window function generation unit 1127 generates a window function (e.g., hanning window, hamming window, Kaiser window, Blackman window or such) used for a fast Fourier transform (FFT) or making a finite impulse response (FIR) filter and outputs the generated window function to the multiplier 1125. The multiplier 1125 multiplies the transmission signal S(t) that is input by way of the delay unit 1121 by the window function input from the window function generation unit 1127 and outputs the transmission signal S(t) by converting it so as to make the amplitude |S(t)| equal to or less than the threshold value Vth.
FIG. 5 shows an original transmission signal, an output signal of a peak suppression circuit of the clip method (noted as “clip method” for convenience hereinafter) and that of a peak suppression circuit of the window function method (noted as “window function method” for convenience hereinafter). Referring to FIG. 5, the solid line curve 1151 is the original transmission signal S(t), and the dashed line curve 1153 on the upper side of the drawing is a transmission signal output from the window function method peak suppression circuit. And the dashed line curve 1155 on the lower side of the drawing is the window function (i.e., a suppressed window function). Here, a=Vth/|S(t)|. As shown in FIG. 5, the suppressed window function is so set that its value varies within “a” to “1”. In the case of the clip method, the amplitude components exceeding the threshold Vth of the original transmission signal S(t) are clipped so as to make the threshold value Vth. In the case of the window function, the original transmission signal S(t) is converted so that the amplitude does not exceed the threshold value Vth.
FIG. 6 shows spectra when applying a clip method and a window function method to an original transmission signal S(t). Referring to FIG. 6, the horizontal axis is frequency and the vertical axis is power (unit: decibel (dB)). The solid line curve 1161 is the spectrum of the original transmission signal S(t), the dotted line curve 1162 is the spectrum of a transmission signal S(t) applied by the clip method and the chain line curve 1163 is that of a transmission signal S(t) applied by the window function method.
In the clip method, only a least necessary transmission signal S(t) is suppressed, reducing the cutoff signal and hence limiting a degradation of a reception characteristic. On the other hand, generating a high frequency component because the edge of suppression is not smooth, hence resulting in degrading greatly the spectrum characteristic as shown in FIG. 6.
In the window function method, a degradation of the spectrum characteristic is smaller compared to the clip method because the edge of suppression can be relatively smooth. In order to limit a degradation of the spectrum characteristic, however, a window function with a long temporal width to some extent must be multiplied to the original transmission signal S(t) and therefore an amount of signal as that much is cut off, enlarging a degradation of the reception characteristic.
Incidentally, though the relationship with the present invention is low, there is a known technique for controlling an average power of a signal input to a transmission amplifier as a technique related to a transmission amplifier (refer to a reference patent document 1).    Patent document 1: Japan Patent Application Laid-Open Publication No. 2002-217828
The above described two methods have characteristics of enabling an implementation by a simple digital signal processing on a transmission side and eliminating a necessity of a specific reproduction process at a reception side. A degradation of a signal quality (i.e., reception quality and spectrum), however, needs to be allowed to some extent because a signal is cut off. That is, the clip method is capable of reducing a degradation of a reception quality, allowing an increase in degradation of a spectrum characteristic. The window function method is capable of reducing a degradation of spectrum characteristic than the clip method, allowing an increased degradation of a reception characteristic.
How much degradation shall be allowed is determined by a requirement of each system or a signal to be transmitted. As an example, the IEEE802.16e (WiMax) Standard has adopted an Adaptive Modulation Coding (AMC) that changes a modulation system and a coding ratio dynamically in accordance with a desired signal quality, with an Error Vector Magnitude (EVM) expressing a distortion of a signal at the time of reception being specified as shown by the table in FIG. 7.
Each row of the table 1170 shown in FIG. 7 consists of three items, i.e., “modulation system”, “coding ratio” and “(permissible) EVM (%)”.
In cases where defining an information bit as x bit and a transmission bit obtained by a coding as y bit, the coding ratio is expressed by:Coding ratio=x/y, 
The table 1170 of FIG. 7 comprehensibly shows that the value of permissible EVM differs greatly depending on modulation system and/or coding ratio. The conventional peak suppression method needs to determine a degree of peak suppression fixedly. Applying a peak suppression of the same degree of suppression to all of various signals as described above and transmitting them with the same back-off, the only possible way is to apply a peak suppression matching with a signal of the strictest standard. This consequently forces a transmission amplifier to operate at a low operating point even where there is no necessity to demand high quality. This in turn requires the transmission amplifier to suppress a transmission power if the same amplifier is to be used; or a bulky, expensive, and large-gain transmission amplifier if it is to be operated at a specified power.